Volatility Question about Mixing Fuels

[Kyle Boatright]

My understanding is that 100LL has a relatively low volatility to reduce the
chance of vapor lock. Whereas Mogas often has a higher volatility.

With this in mind, what happens to the volatility if I mix 50% 100LL with
50% Mogas? Is the volatility of the mix the straight average of the two
fuels, or is the net volatility higher than the straight average?

Why do I ask? I'm considering running a "mix" in the RV-6 to save a few
bucks. The FBO at my home field charges about $2.00/gallon more for 100ll
than the nearby station charges for 93 octane, so a couple of 5 gallon tanks
worth of fuel will save me $20 bucks.

[Bill Daniels]

My experience suggests that the volitility of 100LL is not always less then
mogas.

I have seen 100LL 'fizz' like champagne in a drain test container when mogas
from an airplane parked next to mine showed no volitility at all. This as
at Albuquerque on a 100+ degree afternoon. He departed with no problems.
My fizzing 100LL scared me enough that I stayed overnight and left in the
early morning with cool fuel.

Bill Daniels


"Kyle Boatright" > wrote in message
. ..
> My understanding is that 100LL has a relatively low volatility to reduce
> the chance of vapor lock. Whereas Mogas often has a higher volatility.
>
> With this in mind, what happens to the volatility if I mix 50% 100LL with
> 50% Mogas? Is the volatility of the mix the straight average of the two
> fuels, or is the net volatility higher than the straight average?
>
> Why do I ask? I'm considering running a "mix" in the RV-6 to save a few
> bucks. The FBO at my home field charges about $2.00/gallon more for 100ll
> than the nearby station charges for 93 octane, so a couple of 5 gallon
> tanks worth of fuel will save me $20 bucks.
>
>
>
>
>

[Orval Fairbairn]

In article >,
"Kyle Boatright" > wrote:

> My understanding is that 100LL has a relatively low volatility to reduce the
> chance of vapor lock. Whereas Mogas often has a higher volatility.
>
> With this in mind, what happens to the volatility if I mix 50% 100LL with
> 50% Mogas? Is the volatility of the mix the straight average of the two
> fuels, or is the net volatility higher than the straight average?
>
> Why do I ask? I'm considering running a "mix" in the RV-6 to save a few
> bucks. The FBO at my home field charges about $2.00/gallon more for 100ll
> than the nearby station charges for 93 octane, so a couple of 5 gallon tanks
> worth of fuel will save me $20 bucks.

The volatility should be the average of the two. That said, the octane
of the mix should be significantly higher than the average of the two,
due to the fact that TEL boosts octane nonlinearly -- i.e. small amounts
of TEL boost unleaded fuel, but large amounts do not boost it
proportionately.

[Earl]

On Wed, 04 Apr 2007 03:37:54 GMT, Orval Fairbairn
> wrote:

>In article >,
> "Kyle Boatright" > wrote:
>
>> My understanding is that 100LL has a relatively low volatility to reduce the
>> chance of vapor lock. Whereas Mogas often has a higher volatility.
>>
>> With this in mind, what happens to the volatility if I mix 50% 100LL with
>> 50% Mogas? Is the volatility of the mix the straight average of the two
>> fuels, or is the net volatility higher than the straight average?
>>
>> Why do I ask? I'm considering running a "mix" in the RV-6 to save a few
>> bucks. The FBO at my home field charges about $2.00/gallon more for 100ll
>> than the nearby station charges for 93 octane, so a couple of 5 gallon tanks
>> worth of fuel will save me $20 bucks.
>
>The volatility should be the average of the two. That said, the octane
>of the mix should be significantly higher than the average of the two,
>due to the fact that TEL boosts octane nonlinearly -- i.e. small amounts
>of TEL boost unleaded fuel, but large amounts do not boost it
>proportionately.

So does that mean that it would be OK to use 87 octane mogas because
the octane would be raised by the 100LL?

Steve in Michigan

[Ron Natalie]

Earl wrote:

> So does that mean that it would be OK to use 87 octane mogas because
> the octane would be raised by the 100LL?

Other than the fact that it would put you in violation of the
regulations. The STC's don't have any provision for "custom
blending". You can mix avgas and the minimum octane mogas
(as indicated by your STC) in any ratio you want however.

[Ron Natalie]

Ron Natalie wrote:
> Earl wrote:
>
>> So does that mean that it would be OK to use 87 octane mogas because
>> the octane would be raised by the 100LL?
>
> Other than the fact that it would put you in violation of the
> regulations. The STC's don't have any provision for "custom
> blending". You can mix avgas and the minimum octane mogas
> (as indicated by your STC) in any ratio you want however.
>
Oops...forgot we are talking homebuilts here.

[Orval Fairbairn]

In article >,
Earl > wrote:

> On Wed, 04 Apr 2007 03:37:54 GMT, Orval Fairbairn
> > wrote:
>
> >In article >,
> > "Kyle Boatright" > wrote:
> >
> >> My understanding is that 100LL has a relatively low volatility to reduce
> >> the
> >> chance of vapor lock. Whereas Mogas often has a higher volatility.
> >>
> >> With this in mind, what happens to the volatility if I mix 50% 100LL with
> >> 50% Mogas? Is the volatility of the mix the straight average of the two
> >> fuels, or is the net volatility higher than the straight average?
> >>
> >> Why do I ask? I'm considering running a "mix" in the RV-6 to save a few
> >> bucks. The FBO at my home field charges about $2.00/gallon more for 100ll
> >> than the nearby station charges for 93 octane, so a couple of 5 gallon
> >> tanks
> >> worth of fuel will save me $20 bucks.
> >
> >The volatility should be the average of the two. That said, the octane
> >of the mix should be significantly higher than the average of the two,
> >due to the fact that TEL boosts octane nonlinearly -- i.e. small amounts
> >of TEL boost unleaded fuel, but large amounts do not boost it
> >proportionately.
>
> So does that mean that it would be OK to use 87 octane mogas because
> the octane would be raised by the 100LL?
>
> Steve in Michigan

The answer depends on the minimum octane requirements for your engine.

[Morgans[_2_]]

"Earl" > wrote

> So does that mean that it would be OK to use 87 octane mogas because
> the octane would be raised by the 100LL?

I would think you would want to find how to figure the reslulting blend's
exact octane rating. Otherwise, if it were not high enough, you could end
up creatings some pistons with special "flow through ports" in the piston's
crowns. :-o

Detonation would be a nasty price to pay for saving a couple bucks per tank.
--
Jim in NC

[Capt. Geoffrey Thorpe]

"Orval Fairbairn" > wrote in message
...
> In article >,
> "Kyle Boatright" > wrote:
>
>> My understanding is that 100LL has a relatively low volatility to reduce
>> the
>> chance of vapor lock. Whereas Mogas often has a higher volatility.
<...>
> The volatility should be the average of the two.

I don't know that I would assume that - vapor pressure can have some
non-linear behavior. Example: When you add ethanol, the vapor pressure with
10% is higher than the vapor pressure at either 0% or 85%...

That said, the octane
> of the mix should be significantly higher than the average of the two,
> due to the fact that TEL boosts octane nonlinearly -- i.e. small amounts
> of TEL boost unleaded fuel, but large amounts do not boost it
> proportionately.

Yup.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[jerry wass]

Bill Daniels wrote:
> My experience suggests that the volitility of 100LL is not always less then
> mogas.
>
> I have seen 100LL 'fizz' like champagne in a drain test container when mogas
> from an airplane parked next to mine showed no volitility at all. This as
> at Albuquerque on a 100+ degree afternoon. He departed with no problems.
> My fizzing 100LL scared me enough that I stayed overnight and left in the
> early morning with cool fuel.
>
> Bill Daniels

The volatility-(vapor pressure) of mogas changes with the
season,locatiom & altitude--the plane with the straight mogas probably
resided at Albuq. Whereas avgas keeps pretty much the same volatility,
since it is designed for longer storage times,etc.
>
>
> "Kyle Boatright" > wrote in message
> . ..
>> My understanding is that 100LL has a relatively low volatility to reduce
>> the chance of vapor lock. Whereas Mogas often has a higher volatility.
>>
>> With this in mind, what happens to the volatility if I mix 50% 100LL with
>> 50% Mogas? Is the volatility of the mix the straight average of the two
>> fuels, or is the net volatility higher than the straight average?
>>
>> Why do I ask? I'm considering running a "mix" in the RV-6 to save a few
>> bucks. The FBO at my home field charges about $2.00/gallon more for 100ll
>> than the nearby station charges for 93 octane, so a couple of 5 gallon
>> tanks worth of fuel will save me $20 bucks.
>>
>>
>>
>>
>>
>
>

[GeorgeB]

On Wed, 4 Apr 2007 18:26:52 -0400, "Capt. Geoffrey Thorpe" <The Sea
Hawk at wow way d0t com> wrote:

>> The volatility should be the average of the two.
>
>I don't know that I would assume that - vapor pressure can have some
>non-linear behavior. Example: When you add ethanol, the vapor pressure with
>10% is higher than the vapor pressure at either 0% or 85%...

I am not disagreeing, but do wonder where this information is
available?

Thanks, George

[Capt. Geoffrey Thorpe]

"GeorgeB" > wrote in message
...
> On Wed, 4 Apr 2007 18:26:52 -0400, "Capt. Geoffrey Thorpe" <The Sea
> Hawk at wow way d0t com> wrote:
>
>>> The volatility should be the average of the two.
>>
>>I don't know that I would assume that - vapor pressure can have some
>>non-linear behavior. Example: When you add ethanol, the vapor pressure
>>with
>>10% is higher than the vapor pressure at either 0% or 85%...
>
> I am not disagreeing, but do wonder where this information is
> available?
>
> Thanks, George


I've seen it at work - no help to you, eh?

There are a number of SAE papers covering this. e.g.
http://www.sae.org/technical/papers/852116
http://www.sae.org/technical/papers/2000-01-1712

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.